Over the past two decades Clostridium difficile infection (CDI) has reached epidemic proportions due to emergence of more virulent strains of the pathogen. Fecal microbiota transplantation (FMT) has emerged as a potential solution for many patients refractory to antibiotic treatment alone. We and others have previously shown that FMT promptly restores normal fecal bacterial composition similar to that of the donors. We had also shown that recovery of normal microbial gut ecology is accompanied by major changes in the fecal metabolome, which importantly includes normalization of fecal bile acid composition. The focus on bile acids is mechanistically relevant because they are known to play important roles in the lifecycle of C. difficile. Specifically, taurocholate (a major primary bile acid) is a potent germinant of C. difficile spores, and a critical component of C. difficile growth media in the laboratory. In contrast, lithocholate and deoxycholate (dominant secondary bile acids) are inhibitors of C. difficile spore germination and vegetative growth, respectively. We found that fecal samples from RCDI patients before FMT contain no detectable secondary bile acids, but do have elevated concentrations of primary bile acids. Fecal samples taken from the same patients after FMT contain donor-like fecal bile acid composition. Therefore, our central hypothesis is that antibiotics used to treat CDI patients alter bile acid metabolism to favor C. difficile germination and growth, and FMT restores the normal bile acid composition that is inhospitable to C. difficile expansion. In this proposal we will extend our exploratory preliminary results and test this hypothesis by performing quantitative analysis of bile acid metabolism in patients with RCDI pre- and post-FMT and testing pre- and post-FMT fecal samples for germinant activity and growth inhibition on clinical isolates of C. difficile bacteria. In additionwe will test one non-FMT approach to alter bile acid composition unfavorable to C. difficile: a bile acid sequestrant resin, cholestyramine, in combination with lovastatin. The latter will be used to inhibit compensatory increase in de novo bile acid synthesis in the liver. Results of this work wil inform future clinical trials in treating patients with refractory CDI by targeting bile acid metabolism.